For ascending solution taking a lazy approach, it appears to be a trivial inclusion, considering for any n with a solution using 1-9, a solution which includes

For ascending solution taking a lazy approach, it appears to be a trivial inclusion, considering for any n with a solution using 1-9, a solution which includes 0 will simply take the form: n = 0 + <ascending_solution_n> without any additional consideration. Similarly for descending solution: n = <descending_solution_n> + 0 The descending case also looks like it'll produce a crap ton of solutions for n = 1 of the form: 1 = <arbitrary_descending_permutation> ^ 0